deadlock in example vdpa app

Hi Maintainers,

Recently we see a deadlock in vdpa example app if SIGTERM signal is sent at early start stage. After debuging, we suspect it's due to vhost_user.mutex is requested twice,
once by start sequence start_vdpa(), the other by SIGTERM signal handler which calling close_vdpa().

Also checked other example apps, seems they all have same issue.

Regards
Zhenzhong